The invention relates to a scanning system having a deflectable probe tip with actuators which can be excited to oscillate at or near their resonance frequency.
Scanning systems of the above kind are utilized in optical near-field microscopes as well as in atomic force microscopes; but also in read-write heads for storage devices. When the probe tip approaches the surface of the object or of the storage device, the excited oscillation of the probe tip is influenced because of the atomic interaction between the probe tip and the object surface or surface of the storage device. This oscillation leads to a damping of the oscillation of the probe tip and to a changed phase shift between the oscillation of the probe tip and of the signal exciting the oscillation. This influencing of the oscillation is used as an index for the distance of the probe tip to the object surface or to the surface of the storage device and for the distance control loop of the probe tip. The measurement of the extent of the influence takes place either via a change of the amplitude of the oscillation or via a change of the phase difference between the excitation signal and the oscillation of the probe tip.
A lock-in amplifier is usually used for the above measuring task. Lock-in amplifiers are, however, relatively complex, large, and expensive. This disadvantage is significant especially for scanning systems having a plurality of parallelly-drivable probe tips as described, for example, in U.S. Pat. No. 5,986,262. This situation is present because a corresponding lock-in amplifier is required for each probe tip.
U.S. Pat. No. 5,753,814 discloses an atomic force microscope (AFM) wherein the detected signal is multiplied by the signal of the oscillator, which serves for exciting the oscillation, and the product signal is lowpass filtered to obtain the phase measurement. In a system of this kind, the amplification of the measurement signal must, however, be adapted to the signal intensity of the particular probe tip because of the required detection sensitivity.
It is known from German patent publication 3,050,013 to utilize a phase detector to determine the contact of an oscillation probe pin with the surface of the object being measured in a coordinate measuring apparatus. The phase detector receives, in parallel, the measurement signal for the oscillation of the probe pin and the output signal of the oscillator exciting the oscillation of the probe pin. Information as to the configuration of the phase detector is, however, not contained in this publication except that the phase detector is intended to generate a summation voltage dependent upon the phase difference.
For a probe head of a coordinate measuring apparatus, it is known from U.S. Pat. No. 5,247,751 to generate rectangular signals via waveform shaping from the essentially sinusoidally-shaped measurement signals for the oscillation of the probe head and to generate, via a flip-flop, a pulse-duty factor corresponding to the phase difference between the detected oscillation and the excitation signal. The phase difference is determined via a logic AND coupling of the flip-flop output to clock pulses and subsequent counting.
It is an object of the invention to provide a simple evaluation system for a scanning system having probe tips which is substantially independent of the sensitivity, that is, the signal intensity of the oscillation measurement signal. It is also an object of the invention to provide an evaluation system which is suitable for parallelization and is therefore compact and cost effective.
The scanning system of the invention includes: at least one deflectable probe unit; the probe unit including: a deflectable probe tip having a resonance frequency; and, an actuator operatively connected to the probe tip to impart oscillatory movement thereto; an oscillator for supplying an output excitation signal to the actuator for exciting the probe tip to oscillate at or near the resonance frequency; the probe unit further including a sensor detecting the oscillation of the probe tip and outputting a sensor signal indicative of the oscillation; a detection loop for determining the phase difference between the output excitation signal applied to the actuator and the sensor signal; and, the detection loop including a saturation amplifier for receiving and operating on the sensor signal.
The scanning system of the invention includes a deflectable probe tip as known in scanning systems for near-field microscopes or atomic force microscopes. The deflectable probe tip can be excited to an oscillation at or near its resonance frequency. The deflectable probe tips include actuators for exciting the oscillation. Furthermore, and as in known scanning systems, an oscillator is provided whose output signal is supplied to the actuator and, accordingly, functions to excite oscillation of the probe tip. Further, a detection loop is provided which determines the phase difference between the excitation signal (that is, the output signal of the oscillator) and the detected oscillation carried out by the probe tip. According to the invention, this detection loop includes a saturation amplifier. Such a saturation amplifier, as a rule, comprises several amplifiers connected in series. These amplifiers generate a rectangular signal from an incoming signal which is essentially sinusoidal in shape. This rectangular signal is generated while strictly maintaining the position of the zero crossovers of the input signal except for slight constant time-dependent shifts. From the measurement signal for the oscillation of the probe tip, a rectangularly-shaped signal is accordingly generated having zero crossovers which correspond to the time-dependent position of the zero crossovers of the incoming measurement signal. The subsequent signal evaluation takes place based on a saturation-amplified rectangular signal.
In accordance with the invention, a signal amplification takes place until reaching saturation amplification in advance of further signal evaluation and a further amplification beyond the saturation amplification does not change the signal shape. For this reason, a very high overall amplification can be made available. The reliability of the phase detection is thereby substantially independent of the signal intensity of the measurement signal of the oscillation of the probe tip. Accordingly, the detection loop can be used without further measures for probe tips of the various known configurations. The detection loop is thereby universally useable.
The invention is based on the realization that, for a saturation amplifier, the phase position of the input signal is maintained at the output end of the saturation amplifier substantially independently of the input amplitude over an amplitude dynamic range which is greater than 105. This phase position of the input signal is maintained except for a slight essentially constant phase shift caused by the running time of the amplifier.
In an advantageous embodiment of the invention, the detection loop includes an analog multiplier which is connected downstream of the saturation amplifier. The signal of the oscillator is supplied to the analog multiplier as a second input signal. The oscillator signal, which is supplied to the multiplier, is rectangular to start with or a rectangularly-shaped signal is generated in advance of multiplication from a sine-shaped signal via saturation amplification. The output signal of the analog multiplier is then, in turn, a rectangular-shaped signal, which has a zero crossover each time for a zero crossover of one of the two incoming rectangular-shaped signals. This rectangular signal has twice the frequency of the oscillator signal and a pulse-pause ratio proportional to the phase shift between the oscillator signal and the output signal of the saturation amplifier. For the further signal processing, the multiplier needs only to have a lowpass filter connected downstream thereof whereby a measurement signal is directly present for the phase difference between the oscillating measurement signal and the excitation signal.
To preclude multiple meanings of the measurement signal for the phase difference, it is recommended to provide a second multiplier stage likewise having a lowpass filter connected downstream thereof. This second multiplier stage is supplied with the output signal of the oscillator shifted in phase by 90xc2x0. Based on the overall resulting two-phase measurement values for the phase difference, the phase position between the oscillation and the excitation signal is unequivocally determined.
Alternately to a second multiplier stage, it is in some cases also possible to adjust a base phase relationship with the aid of phase shifters so that no multiple meanings occur, that is, the phase difference is always between 0 and xcfx80 or between xe2x88x92xcfx80 and 0.
The lowpass filter(s) is/are so configured that the double excitation frequency is sufficiently attenuated, for example, by 80 db. This attenuation is in view to the subsequent further processing, such as the subsequent digitalization. Preferably, the excitation frequency is also attenuated.
The arrangement of limit amplifier and analog multiplier is basically known in radio technology as so-called demodulator chips and can be obtained as a mass-produced product. Here, reference is made to the so-called FMIF-system SA 604A of Philips Semiconductors and the corresponding product specification of Nov. 7, 1997. These or similar components furthermore supply a level signal which is proportional to the logarithm of the input signal over a very large dynamic range of 105.
Because of its simplicity, the system of the invention is especially suitable for parallel scanning systems having a plurality of individually and independently controllable probe tips. For each probe tip, an oscillator having a downstream connected power amplifier as well as one or two demodulator chips with saturation amplifier and analog multiplier and lowpass filter is required. For a digital driving and signal evaluation, the corresponding analog-to-digital converters or digital-to-analog converters can be provided in addition for each probe tip.